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      vue不能检测数组和对象的一下变化
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        <h3 id="1、对于数组"><a href="#1、对于数组" class="headerlink" title="1、对于数组"></a>1、对于数组</h3><ul>
<li><p>vue不能检测以下数组的变动</p>
<ul>
<li>1、当你利用索引直接设置一个数组项时：例如 <code>vm.items[indexOfItem] = newValue</code></li>
<li>2、当你修改数组的长度时，例如 <code>vm.items.length = newLength</code></li>
</ul>
</li>
<li><p>例子</p>
</li>
</ul>
<figure class="highlight js"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">var</span> vm = <span class="keyword">new</span> Vue(&#123;</span><br><span class="line">  data: &#123;</span><br><span class="line">    items: [<span class="string">&#x27;a&#x27;</span>, <span class="string">&#x27;b&#x27;</span>, <span class="string">&#x27;c&#x27;</span>]</span><br><span class="line">  &#125;</span><br><span class="line">&#125;)</span><br><span class="line">vm.items[<span class="number">1</span>] = <span class="string">&#x27;x&#x27;</span> <span class="comment">// 不是响应性的</span></span><br><span class="line">vm.items.length = <span class="number">2</span> <span class="comment">// 不是响应性的</span></span><br></pre></td></tr></table></figure>

<ul>
<li>解决方法：</li>
</ul>
<figure class="highlight js"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br></pre></td><td class="code"><pre><span class="line"><span class="comment">// 第一种情况的解决方法：</span></span><br><span class="line">Vue.set(vm.items, indexOfItem, newValue)</span><br><span class="line">Vue.set(vm.items, <span class="number">1</span>, <span class="string">&#x27;x&#x27;</span>)</span><br><span class="line"></span><br><span class="line"><span class="comment">// 第二种情况的解决方法：</span></span><br><span class="line">vm.items.splice(indexOfItem, <span class="number">1</span>, newValue)</span><br><span class="line">vm.items.splice(<span class="number">0</span>, <span class="number">1</span>, <span class="number">2</span>)</span><br></pre></td></tr></table></figure>

<h3 id="1-1、vue中forEach修改数组里的对象视图不刷新，但map却可以"><a href="#1-1、vue中forEach修改数组里的对象视图不刷新，但map却可以" class="headerlink" title="1.1、vue中forEach修改数组里的对象视图不刷新，但map却可以"></a>1.1、vue中forEach修改数组里的对象视图不刷新，但map却可以</h3><figure class="highlight js"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br><span class="line">13</span><br><span class="line">14</span><br><span class="line">15</span><br><span class="line">16</span><br><span class="line">17</span><br><span class="line">18</span><br><span class="line">19</span><br><span class="line">20</span><br><span class="line">21</span><br><span class="line">22</span><br><span class="line">23</span><br><span class="line">24</span><br><span class="line">25</span><br><span class="line">26</span><br><span class="line">27</span><br><span class="line">28</span><br><span class="line">29</span><br><span class="line">30</span><br><span class="line">31</span><br><span class="line">32</span><br><span class="line">33</span><br><span class="line">34</span><br><span class="line">35</span><br><span class="line">36</span><br><span class="line">37</span><br><span class="line">38</span><br><span class="line">39</span><br><span class="line">40</span><br><span class="line">41</span><br></pre></td><td class="code"><pre><span class="line">&lt;template&gt;</span><br><span class="line">    &lt;div @click=<span class="string">&quot;clickHandle&quot;</span>&gt;&#123;&#123;arr[<span class="number">0</span>].value&#125;&#125;&lt;div&gt;</span><br><span class="line">&lt;template&gt;</span><br><span class="line"></span><br><span class="line">&lt;script&gt;</span><br><span class="line"><span class="keyword">export</span> <span class="keyword">default</span> &#123;</span><br><span class="line">    data () &#123;</span><br><span class="line">        <span class="keyword">return</span> &#123;</span><br><span class="line">            arr: []</span><br><span class="line">        &#125;</span><br><span class="line">    &#125;,</span><br><span class="line">    methods: &#123;</span><br><span class="line">        getData () &#123;</span><br><span class="line">        <span class="comment">// 模拟接口返回数据</span></span><br><span class="line">            <span class="keyword">const</span> data = [&#123;</span><br><span class="line">                a: <span class="number">1</span>,</span><br><span class="line">                b:<span class="number">2</span></span><br><span class="line">            &#125;]</span><br><span class="line">            <span class="built_in">this</span>.arr = data</span><br><span class="line">            <span class="built_in">this</span>.arr.forEach(<span class="function"><span class="params">item</span> =&gt;</span> &#123;</span><br><span class="line">                <span class="comment">// 此时发现视图立马更新，div 标签的内容为 false</span></span><br><span class="line">                item.value = <span class="literal">false</span></span><br><span class="line">            &#125;)</span><br><span class="line">        &#125;,</span><br><span class="line">        clickHandle () &#123;</span><br><span class="line">            <span class="comment">// 但是当点击后，标签的值并没有更新，console.log() 打印出来的值发现 value 已经变为了 true</span></span><br><span class="line">            <span class="built_in">this</span>.arr.forEach(<span class="function"><span class="params">item</span> =&gt;</span> &#123;</span><br><span class="line">                item.value = <span class="literal">true</span></span><br><span class="line">            &#125;)</span><br><span class="line">        &#125;</span><br><span class="line">    &#125;</span><br><span class="line">&#125;</span><br><span class="line">&lt;script&gt;</span><br><span class="line"></span><br><span class="line"><span class="comment">// 从上面的例子发现，使用 forEach 给数组的对象添加属性，value 值对应的视图只会在初次渲染的时候时才会更新，点击 click 方法 value 值对应的视图则不会更新，但是变量中的 value 的值已经改变了</span></span><br><span class="line"></span><br><span class="line"><span class="comment">// 问题归总：数据响应式失败</span></span><br><span class="line"></span><br><span class="line"><span class="comment">// 分析：使用 forEach() 遍历数组，给每个数组对象添加一个新的属性 value，这等同于（当你利用索引直接设置一个数组项时：例如 ``` vm.items[indexOfItem] = newValue ```），所以 vue 不能检测到数组的变化，从而导致数据响应式失败</span></span><br><span class="line"></span><br><span class="line"><span class="comment">// 为啥 map() 方法可以呢，因为 map() 会返回一个新的数组，等同于 this.arr = 新数组，所以使用 map() 不会导致数据响应式失败</span></span><br></pre></td></tr></table></figure>


<h3 id="2、对于对象"><a href="#2、对于对象" class="headerlink" title="2、对于对象"></a>2、对于对象</h3><ul>
<li>Vue 无法检测 property 的添加或移除。由于 Vue 会在初始化实例时对 property 执行 getter/setter 转化，所以 property 必须在 data 对象上存在才能让 Vue 将它转换为响应式的。例如</li>
</ul>
<figure class="highlight js"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">var</span> vm = <span class="keyword">new</span> Vue(&#123;</span><br><span class="line">  data:&#123;</span><br><span class="line">    a:<span class="number">1</span></span><br><span class="line">  &#125;</span><br><span class="line">&#125;)</span><br><span class="line"></span><br><span class="line"><span class="comment">// `vm.a` 是响应式的</span></span><br><span class="line"></span><br><span class="line">vm.b = <span class="number">2</span></span><br><span class="line"><span class="comment">// `vm.b` 是非响应式的</span></span><br></pre></td></tr></table></figure>

<ul>
<li>解决方法：</li>
</ul>
<figure class="highlight js"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br></pre></td><td class="code"><pre><span class="line"><span class="comment">// 1、对于已经创建的实例，Vue 不允许动态添加根级别的响应式 property。但是，可以使用 Vue.set(object, propertyName, value) 方法向嵌套对象添加响应式 propert</span></span><br><span class="line"></span><br><span class="line">Vue.set(vm.someObject, <span class="string">&#x27;b&#x27;</span>, <span class="number">2</span>)</span><br><span class="line"></span><br><span class="line"><span class="comment">// 2、还可以使用 vm.$set 实例方法，这也是全局 Vue.set 方法的别名：</span></span><br><span class="line"></span><br><span class="line"><span class="built_in">this</span>.$set(<span class="built_in">this</span>.someObject,<span class="string">&#x27;b&#x27;</span>,<span class="number">2</span>)</span><br><span class="line"></span><br><span class="line"><span class="comment">// 3、有时你可能需要为已有对象赋值多个新 property，比如使用 Object.assign() 或 _.extend()。但是，这样添加到对象上的新 property 不会触发更新。在这种情况下，你应该用原对象与要混合进去的对象的 property 一起创建一个新的对象。</span></span><br><span class="line"></span><br><span class="line"><span class="comment">// 代替 `Object.assign(this.someObject, &#123; a: 1, b: 2 &#125;)`</span></span><br><span class="line"><span class="built_in">this</span>.someObject = <span class="built_in">Object</span>.assign(&#123;&#125;, <span class="built_in">this</span>.someObject, &#123; <span class="attr">a</span>: <span class="number">1</span>, <span class="attr">b</span>: <span class="number">2</span> &#125;)</span><br></pre></td></tr></table></figure>
      
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